Fluid machine including diffuser

ABSTRACT

In a fluid machine, there is provided each diffuser flow passage for making uniform a flow in a downstream of a diffuser. The fluid machine is provided, and it has the diffuser for converting kinetic energy of a fluid into pressure energy. The diffuser has first and second diffuser flow passages configured so that the fluid passes through them, and shapes of the first and second diffuser flow passages are different from each other.

TECHNICAL FIELD

The present invention relates to a fluid machine including a diffuser.

BACKGROUND ART

As a fluid machine including a diffuser, for example, a diffuser pumpthat transports water has been known. Generally, the diffuser pump cangive water kinetic energy by an impeller that is rotatable component ofa diffuser pump, can convert the kinetic energy into pressure energy bya diffuser provided on a discharge side of the impeller, and cantransport the water under high pressure.

As one example, a high-pressure multi-stage diffuser pump includes aplurality of impellers fixed to a rotatable shaft. A diffuser is mountedoutside the impeller of each stage in a radial direction. Diffuser vanesthat define a plurality of diffuser flow passages configured such that afluid discharged from the impellers passes through them are fixed in thediffuser. The fluid having passed through the diffuser flow passages isguided to the impeller of a next stage.

In the diffuser pump, the diffuser is designed so that a pressure lossof the fluid that passes through the pump is reduced, a flow is madeuniform, and that pump efficiency is improved. Conventionally, variousshapes of diffuser flow passages have been developed in order to improvepump efficiency of a diffuser pump (Patent Literature 1). Although adiffuser pump generally includes a plurality of diffuser flow passages,conventional diffuser flow passages all have the same shape.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Laid-Open No. 2013-209883

SUMMARY OF INVENTION Technical Problem

Conventionally, although diffuser flow passages are all designed to havethe same shape, a flow of a fluid discharged from a diffuser is notalways uniform in some cases depending on shapes of flow passageslocated on a downstream of the diffuser. When the flow of the fluiddischarged from the diffuser enters an impeller of a next stage withoutbeing appropriately rectified, pump efficiency may be lowered.

One object of the present disclosure is to provide each diffuser flowpassage for reducing a pressure loss as a whole. In addition, one objectof the present disclosure is to provide each diffuser flow passage formaking uniform a flow on a downstream of a diffuser.

Solution to Problem

According to one aspect of the present disclosure, a fluid machine isprovided, and the fluid machine has a diffuser for converting kineticenergy of a fluid into pressure energy. The diffuser has first andsecond diffuser flow passages configured such that the fluid passesthrough the first and second diffuser flow passages. The first andsecond diffuser flow passages have different shapes.

According to one aspect of the present disclosure, in the fluid machine,each of the first and second diffuser flow passages has an inlet. In atleast a part of each of the first and second diffuser flow passages, across sectional area perpendicular to each of flow passage centers at aposition with an equal distance from each of the inlets of the first andsecond diffuser flow passages is different from each other.

According to one aspect of the present disclosure, in the fluid machine,the fluid machine has a first rotatable impeller for giving the fluidkinetic energy, and the first and second diffuser flow passages arelocated on a downstream of the first impeller in a flow direction of thefluid.

According to one aspect of the present disclosure, in the fluid machine,each of the first and second diffuser flow passages has an outlet. Thefluid machine has: a first confluence flow passage fluidly connected toeach of the outlets of the first and second diffuser flow passages; afirst crossover flow passage fluidly connected to the first confluenceflow passage, the first crossover flow passage running in a direction ofa rotatable shaft of the first impeller; and a first return flow passagefor supplying the fluid to a second impeller of a next stage, the secondimpeller located on a downstream of the first impeller in a flowdirection of the fluid, the first return flow passage being fluidlyconnected to the first crossover flow passage. The first return flowpassage runs in a radially inward direction to a rotatable shaft of thefirst impeller.

According to one aspect of the present disclosure, in the fluid machine,the second diffuser flow passage is located closer to the firstcrossover flow passage than the first diffuser flow passage, and a crosssectional area of the second diffuser flow passage is larger than thatof the first diffuser flow passage.

According to one aspect of the present disclosure, in the fluid machine,the first and second diffuser flow passages are configured such thateach of the cross sectional areas are increased from the inlets towardthe outlets of the respective diffuser flow passages. The seconddiffuser flow passage has a relatively large region, small region, andlarge region in increase rates of the cross sectional areas from theinlet toward the outlet of the diffuser flow passage.

According to one aspect of the present disclosure, in the fluid machine,the diffuser has third and fourth diffuser flow passages configured suchthat the fluid passes through the third and fourth diffuser flowpassages. The third and fourth diffuser flow passages are located on thedownstream of the first impeller in the flow direction of the fluid.Each of the third and fourth diffuser flow passages has an outlet. Thefluid machine has: a second confluence flow passage fluidly connected toeach of the outlets of the third and fourth diffuser flow passages; asecond crossover flow passage fluidly connected to the second confluenceflow passage, the second crossover flow passage running in a directionof a rotatable shaft of the first impeller; and a second return flowpassage for supplying the fluid to the second impeller, the secondreturn flow passage being fluidly connected to the second crossover flowpassage. The second return flow passage runs in a radially inwarddirection to a rotatable shaft of the first impeller.

According to one aspect of the present disclosure, in the fluid machine,the third and fourth diffuser flow passages have shapes of rotationsymmetry of the first and second diffuser flow passages, respectively.

According to one aspect of the present disclosure, in the fluid machine,the third and fourth diffuser flow passages are configured such thateach of the cross sectional areas are increased from inlets toward theoutlets of the respective diffuser flow passages. The fourth diffuserflow passage has a relatively large region, small region, and largeregion in increase rates of the cross sectional areas from the inlettoward the outlet of the diffuser flow passage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing an overall configuration of amulti-stage diffuser pump according to one embodiment;

FIG. 2 is a cross-sectional view of a periphery of impellers anddiffuser vanes of a multi-stage diffuser pump according to oneembodiment;

FIG. 3 is a perspective view of a cross section cut out along a linesegment A-A and a direction of a rotatable shaft of FIG. 2;

FIG. 4 is a view of a cross-section cut out along the line segment A-Aof FIG. 2;

FIG. 5 is a plan view showing a diffuser flow passage according to oneembodiment;

FIG. 6 is a graph showing a relative size of a cross-sectional area ineach position of each diffuser flow passage according to one embodiment;

FIG. 7 is a perspective view of a cross section of a diffuser flowpassage according to one embodiment;

FIG. 8 is a view showing cross-sectional shapes in positions P01 to P06of the diffuser flow passage shown in FIG. 7;

FIG. 9 is a graph showing relative flow rates of a fluid in eachdiffuser flow passage according to one embodiment and each diffuser flowpassage according to a comparative example;

FIG. 10 is a view showing pressure losses of each diffuser flow passageand each confluence flow passage according to one embodiment, and ofeach diffuser flow passage and each confluence flow passage of thecomparative example;

FIG. 11 is a view showing a flow velocity of a fluid in each ofcross-sectional positions P01 to P06 of a diffuser flow passage 104-5according to the comparative example; and

FIG. 12 is a view showing a flow velocity of a fluid in each of thecross-sectional positions P01 to P06 of the diffuser flow passage 104-5according to one embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will beexplained along with accompanying drawings. Note that in theaccompanying drawings, the same symbols are attached to the same orsimilar components, and overlapping explanation is omitted. In addition,features shown in each embodiment can be applied to other embodiments,unless they conflict with each other.

FIG. 1 is a cross-sectional view showing an overall configuration of amulti-stage diffuser pump 1A according to one embodiment of the presentdisclosure. The multi-stage diffuser pump 1A includes a rotatable member30 and a stationary member 40.

The rotatable member 30 includes a rotatable shaft 10 whose both endsare supported. First to seventh impellers I1 to 17 are attached toimpeller attaching parts 10 a to 10 g of the rotatable shaft 10. Therotatable member 30 is mounted rotatably in the stationary member 40.

The stationary member 40 has an outer body part 25. The outer body part25 has a cylindrical member 20 including a suction opening Wi and adischarge opening Wo. In addition, the outer body part 25 has a suctionside plate 18 and a discharge side plate 22 that close both ends of thecylindrical member 20. The stationary member 40 further has an innerbody part 2A. Diffuser vanes V1 to V7 that form pumps P1 to P7 of eachstage along with the impellers I1 to 17 are formed at the inner bodypart 2A.

The first pump P1 is located in a low-pressure chamber R1 thatcommunicates with the suction opening Wi, and includes the impeller I1and the diffuser vane V1. The second to seventh pumps P2 to P7 includethe impellers I2 to I7 and the diffuser vanes V2 to V7. The seventh pumpP7 communicates with a high-pressure chamber R2 that communicates withthe discharge port Wo.

FIG. 2 is cross-sectional view of a periphery of the impellers Il and12, and the diffuser vanes V1 and V2 of a multi-stage diffuser pumpaccording to one embodiment of the present disclosure. In the embodimentshown in FIG. 2, the impellers I1 and I2 fixed to the rotatable shaft 10each have: a plurality of impeller blades 50; a hub 52 at which theimpeller blades 50 have been arranged at equal intervals; and a shroud54 that covers a front surface of the impeller blades 50. A diffuserpart 100 is formed on a downstream side of the impellers I1 and I2,i.e., outside in a radial direction.

FIG. 3 is a perspective view of a cross section cut out along a linesegment A-A and a direction of the rotatable shaft of FIG. 2. FIG. 4 isa view of a cross-section cut out along the line segment A-A of FIG. 2.Note that in FIGS. 3 and 4, the impeller I and the rotatable shaft 10are omitted in order to make clear illustration of the diffuser part100.

As shown in FIGS. 2 and 3, the diffuser part 100 has a plurality ofdiffuser vanes 102. Diffuser flow passages 104 are defined by a wallsurface 109 at a hub 52 side, a wall surface 110 at a shroud 54 side,and each diffuser vane 102, respectively. As will be mentioned in detaillater, each diffuser flow passage 104 is formed so that across-sectional area thereof is increased from an inlet 106 toward anoutlet 108 of the diffuser flow passage 104. In addition, at least somediffuser flow passages 104 have shapes different from each other. Notethat arrows indicate a flow direction of a fluid in FIG. 3.

As shown in FIGS. 3 and 4, a confluence flow passage 150 that fluidlycommunicates with the diffuser flow passage 104 is formed on adownstream side of the outlet 108 of the diffuser flow passage 104,i.e., outside in the radial direction. In the embodiment shown in FIG.4, the four diffuser flow passages 104 fluidly communicate with the oneconfluence flow passage 150, and two sets of the four diffuser flowpassages 104 and the one confluence flow passage 150 are formed. In theillustrated embodiment, the confluence flow passage 150 is located inthe same flat surface as the diffuser flow passage 104. Note that thenumber of the diffuser flow passages 104 and the confluence flowpassages 150 can be arbitrary. For example, in the other embodiment,three diffuser flow passages may fluidly communicate with one confluenceflow passage, and three sets of them may be formed.

The fluid to which kinetic energy has been given by the impeller Il andthat has been discharged enters the diffuser flow passage 104, and thekinetic energy is converted into pressure energy. The fluid having comeout of the outlet 108 of the each diffuser flow passage 104 enters theconfluence flow passage 150 formed on a downstream of the outlet 108 ofthe diffuser flow passage 104. In the diffuser pump according to theembodiment of the present disclosure, shapes of the plurality ofdiffuser flow passages 104 are designed in consideration of a shape ofthe confluence flow passage 150 located on the downstream so that thepressure loss of the fluid discharged from the diffuser flow passages104 is minimized.

In one embodiment, a crossover flow passage 200 that fluidlycommunicates with the confluence flow passage 150 is formed in adownstream of the confluence flow passage 150. In the illustratedembodiment, the crossover flow passage 200 extends in a direction of therotatable shaft 10 as a whole.

In one embodiment, a return flow passage 250 that fluidly communicateswith the crossover flow passage 200 is formed in a downstream of thecrossover flow passage 200. The return flow passage 250 extends inradially inward direction to the rotatable shaft 10 as a whole. Theimpeller I2 of a next stage is formed on a downstream of the return flowpassage 250.

In the illustrated embodiment, the fluid having come out of the impellerI1 passes through the diffuser flow passages 104, and is subsequentlysupplied to the impeller I2 of the next stage through the confluenceflow passage 150, the crossover flow passage 200, and the return flowpassage 250.

As mentioned above, each diffuser flow passage 104 is formed so that thecross-sectional area thereof is increased from the inlet 106 toward theoutlet 108 of the diffuser flow passage 104. In addition, at least somediffuser flow passages 104 have the shapes different from each other.Hereinafter, the shapes of the diffuser flow passages 104 in oneembodiment will be explained in detail.

FIG. 4 is a plan view showing the diffuser flow passages 104 and theconfluence flow passages 150 cut out along the line segment A-A of FIG.2. In the illustrated embodiment, the eight diffuser flow passages 104are defined among the eight diffuser vanes 102. Diffuser flow passages104-1, 104-8, 104-7, and 104-6 fluidly communicate with a confluenceflow passage 150-1. Diffuser flow passages 104-2, 104-3, 104-4, and104-5 fluidly communicate with a confluence flow passage 150-2.Conveniently, the diffuser flow passages 104-1, 104-8, 104-7, and 104-6are set to be a group 1, and the diffuser flow passages 104-2, 104-3,104-4, and 104-5 are set to be a group 2. The fluid having passedthrough the diffuser flow passages 104 of the groups 1 and 2 passesthrough the respective confluence flow passages 150, crossover flowpassages 200, and return flow passages 250, and is supplied to theimpeller of the next stage.

FIG. 5 is a plan view showing one of the diffuser flow passages 104according to one embodiment. As shown in FIG. 5, a curved line thatconnects centers of circles inscribed in the two diffuser vanes 102 isdefined as a flow passage center of the diffuser flow passage 104. Inaddition, a cross section perpendicular to the flow passage centerlocated on an upstream-most side (a left side in FIG. 5) is defined asthe inlet 106 of the diffuser flow passage 104. In addition, a crosssection perpendicular to the flow passage center located on adownstream-most side (a right side in FIG. 5) is defined as the outlet108 of the diffuser flow passage 104.

In the embodiment shown in FIG. 5, a flow passage cross-sectional areaof the diffuser flow passage 104 is increased from the inlet 106 towardthe outlet 108 of the diffuser flow passage 104. In the one embodimentof the present disclosure, at least a part of the plurality of diffuserflow passages 104 in the same group has a shape different from the otherdiffuser flow passages 104 in the same group. More specifically, levelsof increase of the flow passage cross-sectional areas of the diffuserflow passages 104 are different from each other. For example, the areasof the cross sections of the diffuser flow passages 104 perpendicular tothe flow passage center with the same distance from the inlet 106 of theeach diffuser flow passage 104 are different from each other.

In one embodiment, the diffuser flow passage 104 can be configured sothat the level of increase of the flow passage cross-sectional areabecomes larger as the diffuser flow passage 104 is located closer to thecrossover flow passage 200 that fluidly communicates with the confluenceflow passage 150. In the embodiment shown in FIGS. 3 and 4, thecrossover flow passage 200 is located close to the diffuser flowpassages 104-1 and 104-5. Therefore, the diffuser flow passages 104-1and 104-5 close to the crossover flow passage 200 are larger than theother diffuser flow passages 104-2, 104-3, 104-4, 104-6, 104-7, and104-8 in the levels of increase of the flow passage cross-sectionalareas.

FIG. 6 is a graph showing a relative size of a cross-sectional area ofeach position of each of the diffuser flow passages 104-1 to 104-8. Ahorizontal axis represents each of the positions P01 to P06 of thediffuser flow passage shown in FIG. 5. Note that the position P01corresponds to the inlet 106 of the diffuser flow passage 104, and thatthe position P06 corresponds to the outlet 108 of the diffuser flowpassage 104. A vertical axis of the graph of FIG. 6 represents arelative flow passage cross-sectional area in a case where thecross-sectional area of the position P01 of the one diffuser flowpassage 104 used as a comparative example is set to be 100.

In the one embodiment, as for the cross-sectional areas of the diffuserflow passage 104 close to the crossover flow passage 200, the diffuserflow passage 104 has a relatively large region, small region, and largeregion in increase rates of the cross-sectional areas from the inlet 106toward the outlet 108 of the diffuser flow passage 104. For example, inthe graph of FIG. 6, an increase rate of a cross-sectional area of thediffuser flow passage 104-5 close to the crossover flow passage 200 islarge from the positions P01 to P02, becomes relatively smaller from thepositions P02 to P03, and becomes larger again from the positions P03 toP04. The diffuser flow passage 104 is configured as described above, andthereby a mixing loss can be reduced when the fluids from the otherdiffuser flow passages 104 are made to join each other in the confluenceflow passage 150.

As the other embodiment, the diffuser flow passages 104-1, 104-8, 104-7,104-6 of the group 1 and the diffuser flow passages 104-5, 104-4, 104-3,and 104-2 of the group 2 may be formed in shapes of rotation symmetry,respectively.

FIGS. 7 and 8 are views showing one example of cross-sectional shapes ofthe diffuser flow passage 104 according to one embodiment. FIG. 7 is aperspective view of a cross section of the diffuser flow passage 104,and schematically shows the cross-sectional shapes in the positions P01to P06. Note that the diffuser vanes 102 of a front side are shown bybroken lines in FIG. 7. FIG. 8 shows the each cross-sectional shape inthe positions P01 to P06 shown in FIG. 7, respectively. In FIGS. 7 and8, an upper side is the shroud 110 side, and a lower side is the hub 109side.

As shown in FIGS. 7 and 8, in the one embodiment, in the diffuser flowpassage 104, convex portions are provided in the direction of therotatable shaft 10, and sizes of the cross-sectional areas are changed.As shown in FIGS. 7 and 8, in the one embodiment, the wall surface 110at the shroud side of the diffuser flow passage 104 has the convexportion at the positions P01 and P02, the wall surface 109 at the hubside has the convex portion at the position P03, and the wall surfaces109, 110 at the hub side and the shroud side have the convex portions atthe positions P04 to P06. The cross-sectional shape in each position ofthe diffuser flow passage 104 can be arbitrary, and can be set to be adifferent one in the other embodiment. For example, as a non-limitingexample, the cross-sectional shape of the diffuser flow passage 104 canbe set to be an arbitrary one to have the convex portion only on thewall surface 110 at the shroud side, have the convex portion only on thewall surface 109 at the hub side, and to have the convex portions onboth of the wall surfaces 109, 110 at the shroud side and the hub side,from the inlet 106 toward the outlet 108 of the diffuser flow passage104.

EXAMPLE

A graph shown in FIG. 9 shows a result of having determined a flow rateper unit time of each diffuser flow passage by a Computational FluidDynamics (CFD) in a pump including a diffuser flow passage according toone embodiment of the present invention, and a pump including a diffuserflow passage according to a comparative example. In the graph of FIG. 9,a horizontal axis indicates the diffuser flow passages 104-1 to 104-8shown in FIG. 4, and a vertical axis represents a relative flow rate ineach of the diffuser flow passages 104-1 to 104-8. When the relativeflow rate is 1, it means that the fluid with the same flow rate flowsthrough all the diffuser flow passages 104-1 to 104-8. In thecomparative example, all the diffuser flow passages have the same crosssections as those of the comparative example shown in FIG. 6, andconfluence flow passages are the same as those of an example shown inFIG. 9. In the graph of FIG. 9, cross sections of each of the diffuserflow passages 104-1 to 104-8 in the example of the present invention areformed as shown in FIG. 6.

As shown in the graph of FIG. 9, as in the embodiment of the presentdisclosure, the cross-sectional shape is changed for each of thediffuser flow passages 104-1 to 104-8, and thereby variation in the flowrates in the respective diffuser flow passages 104-1 to 104-8 is small.Namely, a mixing loss in the confluence flow passage 150 located on thedownstream of the diffuser flow passage 104 is decreased in theembodiment of the present disclosure, compared with the case of thecomparative example where all shapes of the diffuser flow passages 104are the same as each other.

FIG. 10 is a view showing a result indicating pressure losses of thediffuser flow passage 104 and the confluence flow passage 150 by theabove-described CFD. In FIG. 10, magnitude of the pressure loss is shownby a gray scale, and a deeper gray scale indicates that a largerpressure loss is present. As is understood from FIG. 10, the pressureloss of the embodiment of the present disclosure is smaller as a wholethan that of the case of the comparative example.

FIG. 11 is a view showing a flow velocity of the fluid in each of thecross-sectional positions P01 to P06 of the diffuser flow passage 104-5according to the comparative example. FIG. 12 is a view showing a flowvelocity of the fluid in each of the cross-sectional positions P01 toP06 of the diffuser flow passage 104-5 according to the embodiment ofthe present disclosure. In FIGS. 11 and 12, the flow velocity in each ofthe cross-sectional positions P01 to P06 is shown by iso-flow velocitylines, and the iso-flow velocity lines indicates that the closer to acenter of the cross section, the larger the flow velocity is there. Asis understood from FIGS. 11 and 12, in the embodiment of the presentdisclosure, distortion of the iso-flow velocity line is smaller thanthat of the comparative example, and the graph in FIG. 12 shows adistribution of the flow velocities in which regularly distributedcurves are overlapped. Therefore, in the embodiment of the presentdisclosure, a flow of the fluid that passes through the diffuser flowpassage is uniform, and a rectifying effect is improved. According tothe embodiment of the present disclosure, noise and vibration in thepump can be reduced by enhancing the lowering of the pressure loss andthe rectifying effect.

Although the embodiments of the invention in the present applicationhave been explained as described above, the present invention is notlimited to the above-mentioned embodiments. In addition, respectivefeatures of the above-mentioned embodiments can be combined orexchanged, unless they conflict with each other.

REFERENCE SIGNS LIST

I1 to I7 impeller

100 diffuser part

104 diffuser flow passage

106 inlet of diffuser flow passage

108 outlet of diffuser flow passage

150 confluence flow passage

200 crossover flow passage

250 return flow passage

1-9. (canceled)
 10. A fluid machine comprising: a diffuser forconverting kinetic energy of a fluid into pressure energy, wherein thediffuser has a first diffuser flow passage and a second diffuser flowpassage configured such that the fluid passes through the first andsecond diffuser flow passages, and wherein the first and second diffuserflow passages have different shapes.
 11. The fluid machine according toclaim 10, wherein each of the first and second diffuser flow passageshas an inlet, and wherein in at least a part of each of the first andsecond diffuser flow passages, a cross sectional area perpendicular toeach of flow passage centers at a position with an equal distance fromeach of the inlets of the first and second diffuser flow passages isdifferent from each other.
 12. The fluid machine according to claim 10,wherein the fluid machine has a first rotatable impeller for giving afluid kinetic energy, and wherein the first diffuser flow passage andthe second diffuser flow passage are located on a downstream of thefirst impeller in a flow direction of the fluid.
 13. The fluid machineaccording to claim 11, wherein the fluid machine has a first rotatableimpeller for giving a fluid kinetic energy, and wherein the firstdiffuser flow passage and the second diffuser flow passage are locatedon a downstream of the first impeller in a flow direction of the fluid.14. The fluid machine according to claim 12, wherein each of the firstand second diffuser flow passages has an outlet, the fluid machine has:a first confluence flow passage fluidly connected to each of the outletsof the first and second diffuser flow passages; a first crossover flowpassage fluidly connected to the first confluence flow passage, thefirst crossover flow passage running in a direction of a rotatable shaftof the first impeller; and a first return flow passage for supplying thefluid to a second impeller of a next stage, the second impeller locatedon a downstream of the first impeller in a flow direction of the fluid,the first return flow passage being fluidly connected to the firstcrossover flow passage, and wherein the first return flow passage runsin a radially inward direction to a rotatable shaft of the firstimpeller.
 15. The fluid machine according to claim 13, wherein each ofthe first and second diffuser flow passages has an outlet, the fluidmachine has: a first confluence flow passage fluidly connected to eachof the outlets of the first and second diffuser flow passages; a firstcrossover flow passage fluidly connected to the first confluence flowpassage, the first crossover flow passage running in a direction of arotatable shaft of the first impeller; and a first return flow passagefor supplying the fluid to a second impeller of a next stage, the secondimpeller located on a downstream of the first impeller in a flowdirection of the fluid, the first return flow passage being fluidlyconnected to the first crossover flow passage, and wherein the firstreturn flow passage runs in a radially inward direction to a rotatableshaft of the first impeller.
 16. The fluid machine according to claim14, wherein the second diffuser flow passage is located closer to thefirst crossover flow passage than the first diffuser flow passage, andthe cross sectional area of the second diffuser flow passage is largerthan that of the first diffuser flow passage.
 17. The fluid machineaccording to claim 15, wherein the second diffuser flow passage islocated closer to the first crossover flow passage than the firstdiffuser flow passage, and the cross sectional area of the seconddiffuser flow passage is larger than that of the first diffuser flowpassage.
 18. The fluid machine according to claim 16, wherein the firstdiffuser flow passage and the second diffuser flow passage areconfigured such that each of the cross sectional areas are increasedfrom the inlets toward the outlets of the respective diffuser flowpassages, and wherein the second diffuser flow passage has a relativelylarge region, small region, and large region in increase rates of thecross sectional areas from the inlet toward the outlet of the diffuserflow passage.
 19. The fluid machine according to claim 17, wherein thefirst diffuser flow passage and the second diffuser flow passage areconfigured such that each of the cross sectional areas are increasedfrom the inlets toward the outlets of the respective diffuser flowpassages, and wherein the second diffuser flow passage has a relativelylarge region, small region, and large region in increase rates of thecross sectional areas from the inlet toward the outlet of the diffuserflow passage.
 20. The fluid machine according to claim 14, wherein thediffuser has a third diffuser flow passage and a fourth diffuser flowpassage configured such that the fluid passes through the third andfourth diffuser flow passages, and the third and fourth diffuser flowpassages are located on the downstream of the first impeller in the flowdirection of the fluid, each of the third and fourth diffuser flowpassages has an outlet, the fluid machine has: a second confluence flowpassage fluidly connected to each of the outlets of the third and fourthdiffuser flow passages; a second crossover flow passage fluidlyconnected to the second confluence flow passage, the second crossoverflow passage running in a direction of a rotatable shaft of the firstimpeller; and a second return flow passage for supplying the fluid tothe second impeller, the second return flow passage being fluidlyconnected to the second crossover flow passage, and wherein the secondreturn flow passage runs in a radially inward direction to a rotatableshaft of the first impeller.
 21. The fluid machine according to claim15, wherein the diffuser has a third diffuser flow passage and a fourthdiffuser flow passage configured such that the fluid passes through thethird and fourth diffuser flow passages, and the third and fourthdiffuser flow passages are located on the downstream of the firstimpeller in the flow direction of the fluid, each of the third andfourth diffuser flow passages has an outlet, the fluid machine has: asecond confluence flow passage fluidly connected to each of the outletsof the third and fourth diffuser flow passages; a second crossover flowpassage fluidly connected to the second confluence flow passage, thesecond crossover flow passage running in a direction of a rotatableshaft of the first impeller; and a second return flow passage forsupplying the fluid to the second impeller, the second return flowpassage being fluidly connected to the second crossover flow passage,and wherein the second return flow passage runs in a radially inwarddirection to a rotatable shaft of the first impeller.
 22. The fluidmachine according to claim 16 wherein the diffuser has a third diffuserflow passage and a fourth diffuser flow passage configured such that thefluid passes through the third and fourth diffuser flow passages, andthe third and fourth diffuser flow passages are located on thedownstream of the first impeller in the flow direction of the fluid,each of the third and fourth diffuser flow passages has an outlet, thefluid machine has: a second confluence flow passage fluidly connected toeach of the outlets of the third and fourth diffuser flow passages; asecond crossover flow passage fluidly connected to the second confluenceflow passage, the second crossover flow passage running in a directionof a rotatable shaft of the first impeller; and a second return flowpassage for supplying the fluid to the second impeller, the secondreturn flow passage being fluidly connected to the second crossover flowpassage, and wherein the second return flow passage runs in a radiallyinward direction to a rotatable shaft of the first impeller.
 23. Thefluid machine according to claim 17, wherein the diffuser has a thirddiffuser flow passage and a fourth diffuser flow passage configured suchthat the fluid passes through the third and fourth diffuser flowpassages, and the third and fourth diffuser flow passages are located onthe downstream of the first impeller in the flow direction of the fluid,each of the third and fourth diffuser flow passages has an outlet, thefluid machine has: a second confluence flow passage fluidly connected toeach of the outlets of the third and fourth diffuser flow passages; asecond crossover flow passage fluidly connected to the second confluenceflow passage, the second crossover flow passage running in a directionof a rotatable shaft of the first impeller; and a second return flowpassage for supplying the fluid to the second impeller, the secondreturn flow passage being fluidly connected to the second crossover flowpassage, and wherein the second return flow passage runs in a radiallyinward direction to a rotatable shaft of the first impeller.
 24. Thefluid machine according to claim 18, wherein the diffuser has a thirddiffuser flow passage and a fourth diffuser flow passage configured suchthat the fluid passes through the third and fourth diffuser flowpassages, and the third and fourth diffuser flow passages are located onthe downstream of the first impeller in the flow direction of the fluid,each of the third and fourth diffuser flow passages has an outlet, thefluid machine has: a second confluence flow passage fluidly connected toeach of the outlets of the third and fourth diffuser flow passages; asecond crossover flow passage fluidly connected to the second confluenceflow passage, the second crossover flow passage running in a directionof a rotatable shaft of the first impeller; and a second return flowpassage for supplying the fluid to the second impeller, the secondreturn flow passage being fluidly connected to the second crossover flowpassage, and wherein the second return flow passage runs in a radiallyinward direction to a rotatable shaft of the first impeller.
 25. Thefluid machine according to claim 10, wherein the diffuser has a thirddiffuser flow passage and a fourth diffuser flow passage configured suchthat the fluid passes through the third and fourth diffuser flowpassages, and the third and fourth diffuser flow passages are located onthe downstream of the first impeller in the flow direction of the fluid,each of the third and fourth diffuser flow passages has an outlet, thefluid machine has: a second confluence flow passage fluidly connected toeach of the outlets of the third and fourth diffuser flow passages; asecond crossover flow passage fluidly connected to the second confluenceflow passage, the second crossover flow passage running in a directionof a rotatable shaft of the first impeller; and a second return flowpassage for supplying the fluid to the second impeller, the secondreturn flow passage being fluidly connected to the second crossover flowpassage, and wherein the second return flow passage runs in a radiallyinward direction to a rotatable shaft of the first impeller.
 26. Thefluid machine according to claim 11, wherein the third diffuser flowpassage and the fourth diffuser flow passage have shapes of rotationsymmetry of the first diffuser flow passage and the second diffuser flowpassage, respectively.
 27. The fluid machine according to claim 12,wherein the third diffuser flow passage and the fourth diffuser flowpassage have shapes of rotation symmetry of the first diffuser flowpassage and the second diffuser flow passage, respectively.
 28. Thefluid machine according to claim 13, wherein the third diffuser flowpassage and the fourth diffuser flow passage have shapes of rotationsymmetry of the first diffuser flow passage and the second diffuser flowpassage, respectively.
 29. The fluid machine according to claim 14,wherein the third diffuser flow passage and the fourth diffuser flowpassage have shapes of rotation symmetry of the first diffuser flowpassage and the second diffuser flow passage, respectively.
 30. Thefluid machine according to claim 15, wherein the third diffuser flowpassage and the fourth diffuser flow passage have shapes of rotationsymmetry of the first diffuser flow passage and the second diffuser flowpassage, respectively.
 31. The fluid machine according to claim 16,wherein the third diffuser flow passage and the fourth diffuser flowpassage have shapes of rotation symmetry of the first diffuser flowpassage and the second diffuser flow passage, respectively.
 32. Thefluid machine according to claim 11, wherein the third diffuser flowpassage and the fourth diffuser flow passage are configured such thateach of the cross sectional areas are increased from inlets towardoutlets of the respective diffuser flow passages, and wherein the fourthdiffuser flow passage has a relatively large region, small region, andlarge region in increase rates of the cross sectional areas from theinlet toward the outlet of the diffuser flow passage.
 33. The fluidmachine according to claim 12, wherein the third diffuser flow passageand the fourth diffuser flow passage are configured such that each ofthe cross sectional areas are increased from inlets toward outlets ofthe respective diffuser flow passages, and wherein the fourth diffuserflow passage has a relatively large region, small region, and largeregion in increase rates of the cross sectional areas from the inlettoward the outlet of the diffuser flow passage.
 34. The fluid machineaccording to claim 13, wherein the third diffuser flow passage and thefourth diffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 35. The fluid machine according toclaim 14, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 36. The fluid machine according toclaim 15, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 37. The fluid machine according toclaim 16, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 38. The fluid machine according toclaim 17, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 39. The fluid machine according toclaim 18, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 40. The fluid machine according toclaim 19, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 41. The fluid machine according toclaim 20, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 42. The fluid machine according toclaim 21, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.
 43. The fluid machine according toclaim 22, wherein the third diffuser flow passage and the fourthdiffuser flow passage are configured such that each of the crosssectional areas are increased from inlets toward outlets of therespective diffuser flow passages, and wherein the fourth diffuser flowpassage has a relatively large region, small region, and large region inincrease rates of the cross sectional areas from the inlet toward theoutlet of the diffuser flow passage.